Development of optical modules using Si photonic elements such as, for example, an optical modulator, an optical receiver, and a coherent optical sub-assembly (COSA) obtained by combing the optical modulator and the optical receiver is now popular. The Si photonic element is formed of a lightwave circuit in which fine optical waveguides are formed, the optical waveguides using, as a core material, silicon which is widely used as a semiconductor and which has a far greater refractive index than silicon oxide films and polymers. The optical waveguide using the Si core has a relative refractive index ten times or more than that of a conventional PLC, a mode field diameter (MFD) 1/10 or less thereby achieving strong confinement in a micron order, and a small minimum bend radius (5 μm or less). Accordingly, the Si photonic element has a characteristic of being far smaller than the conventional optical device, and an optical module such as an optical transceiver which has a smaller size and a higher density packaging than the conventional optical device is expected to be achieved.
This Si photonic element and small optical parts are usually used in combination with other optical fiber pigtail modules such as an LD module. There is a demand for size reduction not only for each of these modules but also for an entire sub-board for connecting the modules with optical fibers and packaging them in combination or an entire case for housing the modules such as, for example, a case with size restriction such as CFP2.
The Si cores of the optical waveguides in the Si photonic element are finer than cores formed by using a SiO2 film and are formed in a square each side of which is in sub-micron order. When such fine optical waveguides are simply connected to normal optical fibers, the optical coupling loss is large. Accordingly, the mode field diameter of light emitted from an end of each optical waveguide needs to be increased.
Specifically, in the Si photonic lightwave circuit, optical waveguides are integrated with the radius of curvature being about 5 and the functions of the circuit are achieved in a smaller space. In addition, more and more optical fibers and electric circuits are employing multi-array designs. However, the radius of curvature of the optical fibers is about, for example, 15 mm. Accordingly, it is necessary to reduce not only the size of the Si photonic lightwave circuit but also the size of a module group including routed portions of external optical fibers.
FIG. 11 illustrates a typical example of a conventional Si photonic element. Two package modules 1102, 1105 are mounted on the same board 1101. In this example, a Si photonic lightwave circuit 1103 is 20 mm square and is mounted on the 30 mm square package module 1102. Moreover, the Si photonic lightwave circuit 1103 is connected to an optical fiber array 1106 fixed by an optical fiber two-core ferrule (8×10 mm) 1104 which is fixed on a right side of the Si photonic lightwave circuit 1103 in the drawing. Methods of connecting a two-core optical waveguide of the Si photonic lightwave circuit 1103 to the optical fiber array 1106 include a method of fixing the optical fiber ferrule with YAG welding while interposing a lens, a method of fixing the metal-coated optical fiber with solder, and the like.